A method for diagnosing insulation degradation of a power cable is disclosed in Japanese Examined Patent Publication No. H 06-7146. The method diagnoses insulation degradation of a power cable using discharge current caused by a partial discharge. As shown in FIG. 1A, a diagnostic device 101 comprises a cable 102, an intermediate connecting section 103, terminal joints 104a and 104b, a high frequency blocking coil 105, a high voltage power source 106, a coupling capacitor 107, a detecting impedance 108, and a partial discharge measurement device 109. Here, FIG. 1A and FIG. 1B are a wiring diagram and an equivalent circuit diagram of the diagnostic device 101, respectively.
The cable 102 is configured by connecting two cables using the intermediate connecting section 103. Further, the cable 102 has a first end portion connected to a first end portion of the terminal joint 104a and a second end portion connected to a first end portion of the terminal joint 104b. The terminal joint 104a has a second end portion connected to the high voltage power source 106 via the high frequency blocking coil 105. The terminal joint 104b has a second end portion connected to the detecting impedance 108 via the coupling capacitor 107. The partial discharge measurement device 109 has both ends respectively connected to both ends of the detecting impedance 108 and detects a potential difference that is generated across both ends of the detecting impedance 108. A capacitor 102a in FIG. 1B has an electrostatic capacitance equal to the electrostatic capacitance of the cable 102.
Next, an insulation degradation diagnostic method of the diagnostic device 101 will be described. After the operation of the cable 102 is stopped, a test voltage is applied to the cable 102 from the high voltage power source 106. By this operation, a partial discharge occurs in an insulator of the cable 102 and discharge current is induced in a conductor of the cable 102. Incidentally, the discharge current has a high frequency pulse waveform. The discharge current is output to the detecting impedance 108 via the coupling capacitor 107. The partial discharge measurement device 109 detects a pulse voltage that is generated across both ends of the detecting impedance 108 and generates data. After performing predetermined processing to the generated data, the insulation degradation of the cable 102 is diagnosed.
The partial discharge measurement device 109 is, for example, a tuning type partial discharge measurement device. The partial discharge measurement device 109 comprises a tuning detector, a wide band attenuator, a tuning amplifier, a detector, etc. (none of them is shown). The tuning detector detects a pulse voltage as a waveform of constant frequency attenuation oscillation. The wide band attenuator attenuates an output waveform of the tuning detector to a proper level. The tuning amplifier tunes and amplifies the output waveform of the wide band attenuator at a tuning frequency with “400” kHz being as its center in order to avoid the radio broadcast band. The detector detects the output waveform of the tuning amplifier.
A method for diagnosing insulation degradation of a branch joint of a power distribution high voltage overhead cable is disclosed in Japanese Patent Application Publication No. 2000-2743. The method diagnoses insulation degradation of a branch joint by utilizing a high frequency current flowing through a lead-in cable.
As shown in FIG. 2, a lead-in cable 202 is branched from an overhead cable 208 by a joint 203. A diagnostic device 201 comprises a current transformer 204, an amplifier 205, a spectrum analyzer 206, and a computer 207. The current transformer 204 is attached to the lead-in cable 202. The amplifier 205 amplifies the waveform of a high frequency current detected by the current transformer 204. The spectrum analyzer 206 measures the frequency spectrum of the amplified high frequency current. The computer 207 stores the waveform pattern and the frequency spectrum of a high frequency current in a memory.
Next, an insulation degradation diagnosing method of the diagnostic device 201 will be explained. The reference waveform pattern and the reference frequency spectrum of a high frequency current in the normal joint 203 are stored in advance in the memory of the computer 207. Next, the waveform pattern and the frequency spectrum of a high frequency current in the joint 203 to be diagnosed is stored in the memory of the computer 207 and compared with the reference waveform pattern and the reference frequency spectrum, respectively. By this comparison, the degree of partial discharge, that is, the degree of insulation degradation in the joint 203 is diagnosed.
When a partial discharge has occurred in the joint 203, in bands of 2 to 6 MHz and 6 to 10 MHz, the frequency spectrum of the high frequency current takes a large value. The frequency spectrum with general environmental noises takes a small value in the bands of 2 to 6 MHz and 6 to 10 MHz, therefore, the occurrence of partial discharge is easily discriminated from the occurrence of environmental noises by the above-mentioned method.
However, with the former diagnostic method, it is necessary to attach the coupling capacitor 107 to the cable 102 and stop the operation of the cable 102. Therefore, this diagnostic method is used only for inspection on shipping of the product or the characteristic evaluation at the time of development of the product. Further, since a single frequency (400 kHz) is extracted from the pulse voltage detected with the detecting impedance 108 and used for the partial discharge measurement, this method is not practical. Furthermore, the partial discharge measurement device 109 is a tuning type partial discharge measurement device etc., therefore, the circuit configuration becomes more complex.
With the latter diagnostic method, since a spectrum analyzer is used to obtain the frequency spectrum of a high frequency current, the device becomes more expensive.